What is the underlying grammar in the difference between `足らない` and `足りない`?

Question

Based on the comments from Tsuoyshi Ito's answer to Translation of 竜すらも恐るるに足らない, 恐るるに足らない/恐るるに足りない is a set phrase meaning "not worth fearing".

Based on my understanding of 一段 verbs like 足りる, negation will yield 足りない.

How do I get to 足らない ?

Answer 1

There are two verbs: a 五段 verb 足る and an 一段 verb 足りる. They both have the same meaning. According to the Daijisen [1], 足る was the original, and 足りる started being used in the early modern / Edo era:

四段活用の「足る」から転じ、近世から江戸で用いられるようになったもの。